The mode-locking of a lasers to produce short pulses of light is known in the prior art. A mode of a laser refers to a discrete wavelength which can propagate in the laser cavity, because of the standing wave conditions for a linear cavity or because of phase matching conditions for a ring laser. The process of short pulse generation relies on the modes of a laser being locked in phases at some point in the cavity, such that the interference of these modes with each other produces a short duration pulse when the phases add constructively, and no pulse when the modes add destructively. The width of the pulse is related to the number of modes which can be locked together. Mode locking can be classified as either passive or active.
Active mode-locking of a laser occurs when the light in the cavity is actively modulated in such a way as to induce a phase modulation. This can be achieved by gain, loss or phase modulation--for example, amplitude modulation at a given frequency will set up side bands for a given mode. The modulation frequency can be chosen such that the frequencies of these sidebands correspond exactly to the frequencies of the neighbouring modes, which then will assume a phase which is directly related to the first mode. The second mode can the phase-lock in the same fashion with another neighbouring mode. This process can continue until all the modes which receive sufficient gain are phase-locked together. The result is a short pulse of light when the modes are in phase. An alternative way of considering gain (or loss) modulation mode-locking is in terms of the temporal narrowing of pulses after each successive pass through the laser. Upon each transit of the light through the modulator, the peak transmission coincides with the point of maximum intensity. The edges of the pulses are reduced with respect to the peak, and the pulse is narrowed. The process can continue until the narrowing due to the modulation is counteracted by other broadening mechanisms occurring in the loop.
Passive mode-locking of a laser is similar to mode-locking through gain modulation, but in this case the modulation is not provided externally. Passive modelocking is due to the action of a passive component upon the shape of a pulse which is traveling through the laser. Passive mode-locking has been achieved by the insertion of a saturable absorber into the cavity. A saturable absorber is a medium which at the wavelength of interest has an intensity dependent absorption characteristic. At low powers, the absorption is strong, but for higher powers the absorption is reduced due to saturation of the absorbing medium. A pulse passing through a saturable absorber will thus experience greater absorption at the tails of the pulse than at the peak. This allows pulse narrowing to take place, and the gain modulation in turn will permit mode-locking of the laser. This results in a self-pulsating laser. This process can be initiated either by the injection of a pulse, or by the build up of random fluctuations into a pulse. Once initiated, it should continue indefinitely until something is done to disturb the system.
One limitation on prior art modelocking techniques is the speed with which the saturable absorber can achieve saturation. This is not an instantaneous effect, and so limits the temporal direction of the pulses to be wider than some value irrespective of the number of modes under the gain spectrum. Finding a suitable saturable absorber at the wavelength of interest and with the necessary response times limits the systems which can be developed. This is a particular difficulty in fibre systems.